The invention relates to distillation systems generally and particularly to distillation systems of the "flash" variety where pure water is recovered from sea water, from other solute-laden water, and/or from waters contaminated with various chemical and/or physical pollutants. The invention can further be used as a distiller-concentrator to produce an effluent which is more concentrated in a given solute.
Flash distillation apparatus of the type described have generally been constructed with flat steel sheets enclosing heat exchange apparatus. Materials such as copper, nickel or titanium, while desirable as materials for use in such structures, require uneconomical heavy wall construction when used as flat sides in apparatus of this type. Steel stiffeners must also be employed, requiring welding of dissimilar metals, even at relatively low operating pressures. The present apparatus is particularly suited to use of these relatively exotic and expensive materials since a minimum of the material need be used to provide a structure with sufficient integrity to withstand the increasingly high pressures and operating temperatures employed not only in present-day distillation systems, but also in presently experimental systems which will likely be "on-line" in the near future. A variety of materials may be used in the fabrication of the present apparatus and, due to the particular structure thereof, these materials may be mixed without disadvantageous galvanic effects, without assembly difficulties, and without compromise of structural integrity. Such materials include but are not limited to various plastics, reinforced fiber-glass, and presently available coated steels. It is of particular note that the present invention provides economies of construction, not only in pecuniary terms but also in terms of efficient use of materials, which also provide operating economies in terms of product output per unit of power input.
Advancing process technologies such as the method of preventing scale information at high operational temperatures disclosed by E.A. Cadwallader and me in co-pending U.S. patent application Ser. No. 503,108 now abandoned, increase corrosion in steel enclosure vessels despite the use of coatings. Thus, the ability to use economically materials such as titanium to contact sea water as is provided by the present invention is of great importance in the art. The invention further provides the advantages of being easily combinable into multi-effect distillation systems of widely varying capacities and of being volumetrically efficient. The "back-to-back" arrangement of the flash chambers comprising the present apparatus allows a compact horizontal arrangement requiring substantially less space than conventional vertically disposed, multichambered distillation apparatus of similar capacity. A further advantage in material usage and economy of operation of the inventon lies in conforming the present evaporating chambers without heat exchange sections in the chambers. Thus, heating coils such as are disclosed by me in U.S. Pat. No. 3,302,373 need not be disposed in the present evaporating chambers.
The higher operating pressures permitted by the present cylindrically conformed apparatus coupled with the fact that said apparatus is in contact with sea water requires the use of expensive and strategically critical materials in the fabrication of the apparatus. When compared to present day rectangular evaporator construction, the design of the present apparatus greatly reduces weight and material cost. The higher operating pressure also makes possible external ducting of the vapor (due to the greatly reduced volume of the evaporator) to a composite heat exchanger, thereby resulting in a 40% saving over the cost of separate individual staged heat exchangers embodied in the flash chambers of prior designs.
The invention can further be used to provide a "topping effect" to existing low temperature distilling plants. Present distillers operate between a top temperature of 190.degree. F. and a low temperature of 100.degree. F. The present horizontally-disposed cylindrical back-to-back configuration can be coupled in series (MSF), in parallel (MEF), or both (MEMS) to optimize stages and/or effects and also to operate between at least 285.degree. F. and 195.degree. F. Thus, the operating temperature range is doubled, the efficiency of existing plants being more than doubled. The additional capital investment would be amortized quickly at the present cost of energy.
The invention further allows the use of light gauge, "non-pressure" parts for fabrication of a composite flash and mesh separator device. Due to the essentially unitary construction of this composite device, repair or replacement is quickly accomplished by simple unbolting of the device from the evaporator chamber. Further, if scaling occurs, all components of the evaporator can be easily disassembled for cleaning. In prior art evaporators, the flash device is an integral part of the flash chamber. Thus, the only way to make repairs is by cutting through the pressure enclosure or by providing access doors in small plants or man-ways in large plants. All of these prior practices have proven costly and time-consuming.
The invention further provides a flash device which exposes both the top and bottom of the "sheet" of sea water or bine to evaporation, thus having the effect of reducing the size of the flash chamber in half for the same release rate in pounds of vapor per square foot of surface exposed. In prior art flash evaporators, only the top surface is exposed to evaporation.
The present apparatus further acts to reduce the size of the mesh separator by interposing several reversals of flow in order to separate water droplets from vapor. These several changes of direction also has the effect of reducing not only the "escape height" (which increases the normal height of the plant) but also the size of the mesh necessary for effective separation of brine from vapor since water loading on the mesh is reduced. The mesh is located directly above the flashing pool of liquid in present designs, direct impingement (especially with foaming liquids) of water droplets on the mesh therefore occurring.
The present invention also utilizes a division plate between stages which is exposed to only pure vapor. Since a temperature gradient exists between stages, the contact of the division plate with only pure vapor prevents deposition of scale on the division plate. In prior evaporators, the dividing structure between "hot" and "cold" chambers lies in the brine flow path, the brine on the "colder" side hitting the dividing structure and evaporating to deposit salt and scale which "creeps" up the hot surface, by-passes the separators, and thus causes carry-over of impurities. The present conformation is particularly useful with the use of CO.sub.2 as a scale inhibitor. In prior evaporators, the use of CO.sub.2 would necessitate the use of double, insulated walls between stages, thereby resulting in a more costly structure.
Accordingly, it is an object of the invention to provide evaporator-concentrator apparatus which can be economically fabricated from a variety of materials, including relatively expensive corrosion-resistant metals.
It is a further object of the invention to provide multi-effect distillation apparatus having horizontally disposed, back-to-back evaporating chambers.
It is another object of the invention to provide material and energy efficient distillation apparatus which is combinable into distillation systems of varying capacity with maximal volumetric efficiency.
Further objects and advantages of the invention will become apparent in light of the following detailed description of the preferred embodiments of the invention.